I saw this simple tip in Petapixel. I think it's a way to see what happens with different colours when we convert to Black and White, but also handy if you want to alter specific colours in an image.Open your image in PhotoSshop CS or Elements1. Click on B&W Icon and select Black and White Adjustment Layer 2Change the Blending Mode to Luminosity3. Play with colour-sliders to your hearts delightFortunately we automatically have a mask here, so paint with Black on mask to remove unwanted changes in your image

P.S. if you're in the mood for reading something that uses the (notorious) passport and seems like it will be enlightening but remains (to me at least) inscrutable after several readings (how's that for an invitation!), see:

Very interesting. If I had to work these mechanisms and workings out, we would only have abstract images :-)I really enjoy the passport peeping out of the images. Must say, I'm quite impressed with the colorchecker system.Makes application of CM easier in some instances

Art, you take an image with the colorchecker passport in (must fill at least 10% of the frame). Open that image in Lightroom, or ACR (In PS or Elements) From within these programs, export to the X-Rite software, which recognizes the passport and write a colour-profile back into ACR or Lightroom, based on the recognition of the colour-shift the clever software "see" on the image. By applying this profile to your image or all images that you took under the same light condition, will fix the colour cast. I checked it with CM afterwards and the neutrals are neutral. The software base the profile on metadata from the image, thus it is for a specific camera and lens under specific conditions. There are demos on YouTube